Fully dense consolidated-powder superalloys

ABSTRACT

SUPERALLOYS FORMED OF CONSOLIDATED POWDER, WHICH ARE FULLY DENSE AND HAVE A COMPOSITION BY WEIGHT CONSISTING ESSENTIALLY OF ABOUT 0.08 TO 0.20 PERCENT CARBON, ABOUT 9 TO 34 PERCENT COBALT, ABOUT 8 TO 17 PERCENT CHROMIUM, ABOUT 2.5 TO 6 PERCENT ALUMINUM, ABOUT 1.7 TO 5 PERCENT TITANIUM, ABOUT 6.5 TO 14 PERCENT TUNGSTEN AND THE BALANCE NICKEL TOGETHER WITH MINOR ALLOYING INGREDIENTS AND INCIDENTAL IMPURITIES, ARE CHARACTERIZED BY BEING READILY WORKABLE AS COMPARED TO CONVENTIONAL CASTING ALLOYS OF THE SAME CHEMICAL COMPOSITION WHICH ARE TOO BRITTLE AND HAVE TOO LITTLE DUCTILITY TO HOT WORK.

3,681,061 FULLY DENSE CONSOLIDATED-POWDER SUPERALLOYS Stewart G. Fletcher, Latrobe, Pa., assignor to Latrobe Steel Company, Latrobe, Pa. No Drawing. Filed Feb. 16, 1970, Ser. No. 11,861 Int. Cl. C22c 19/00 U.S. Cl. 75-171 6 Claims ABSTRACT OF THE DISCLOSURE Superalloys formed of consolidated powder, which are fully dense and have a composition by weight consisting essentially of about 0.08 to 0.20 percent carbon, about 9 to 34 percent cobalt, about 8 to 17 percent chromium, about 2.5 to 6 percent aluminum, about 1.7 to percent titanium, about 6.5 to 14 percent tungsten and the balance nickel together with minor alloying ingredients and incidental impurities, are characterized by being readily workable as compared to conventional casting alloys of the same chemical composition which are too brittle and have too little ductility to hot work.

The present invention relates to a workable nickel base superalloy and more particularly to a heat-resistant, high-strength, structural alloy of fully dense consolidated powder prepared from solidified powder particles featuring an ultrafine microstructure in which the secondary dendrite arm spacing is substantially all less than about 0.0003 inch and in which the microsegregation of the consolidated alloy has been reduced to limits heretofore unattainable.

The utilization of high temperatures for many diverse operations has become an accepted part of the technology of modern industrial processes. Also, the quest for improved power sources has led to the development of such devices as superchargers, gas turbines, jet engines and the like all operating at elevated temperatures. These developments demand metals and alloys which will withstand prolonged exposure to temperatures well above about 1300 F. and in many instances well above about 1700 F., and are capable of withstanding severe mechanical stress at these temperatures. In many instances it is desired that alloys for use in such apparatus be capable of being hot-worked and machined, while in other instances the alloys may be employed in the form of castings. In any event such alloys must have high strength in order to be useful.

At the present time a number of relatively highly alloyed nickel base alloys, commonly referred to in the trade as superalloys, are commercially available. One such alloy commonly referred to as Mar-M-200 has the following composition: about 0.15% carbon, about cobalt, about 9% chromium, about 5% aluminum, about 2% titanium, about 12.5% tungsten, about 1% columbium, about 0.013% boron, about 0.05% zirconium and the balance nickel with usual impurities. A similar commercial alloy known as Nicrotung has the following composition: about 0.10% carbon, about 10% cobalt, about 12% chromium, about 4% aluminum, about 4% titanium, about 8% tungsten, about 0.05% zirconium, about 0.05 boron, and the balance nickel with the usual impurities in ordinary amounts. Commercial alloys such as Mar-M- 200 and Nicrotung have good oxidation resistance and retain fairly good strength values up to temperatures of 1800 F. or even in some cases up to 2000 F. These nickel base alloys are casting alloys which have relatively low ductility and accordingly are used in their as-cast shape.

Presently as-cast nickel base alloys which correspond 3,681,061 Patented Aug. 1., 1972 compositionally to the alloys in the novel form of the present invention are characterized by relatively coarse dendritic structures which seriously detract from the physical and metallurgical properties of the materials. Because of this undesirable microstructure these alloys are brittle and extremely difiicult to fabricate into useful shapes.

The present invention provides nickel base alloys in a novel form which overcome numerous shortcomings and disadvantages of previously known alloys and which have particular utility at high temperatures. Such alloys are considerably stronger than presently available cast alloys of similar chemical composition, but at the same time are readily fabricable into useful Wrought forms.

Alloys of the present invention can have the general composition range of:

Percent by wt.

Balance substantially nickel with residual impurities in ordinary amounts.

Preferred compositions for most applications in accordance with the present invention have the composition range given below for alloys I and II:

Percent by weight I II Cb Balance substantially nickel with residual impurities in ordinary amounts.

The expression less than followed by a percentage figure for silicon, manganese, iron, zirconium, boron and columbium means that the particular element may be entirely absent or may be present up to the concentration given without deleteriously affecting the alloy.

Each of the superalloys provided by the present invention, though compositionally similar to certain alloys of the prior art, can be distinguished therefrom by its novel metallographic structure with the attendant increase in desirable mechanical properties. More specifically, each of the present nickel-base alloys is characterized by being readily workable.

Because of the high ductility of the unique ultrafine microstructure, the nickel base alloys of this invention can be readily fabricated into useful shapes and retain a large amount of the cold work induced during fabrication which improves the strength of the alloys.

In accordance with the present invention an atomized, prealloyed powder of the desired composition is first made by atomizing a molten alloy charge consisting essentially of the ingredients in substantially the proportions stated in the general compositions set out hereinabove. The molten alloy charge can be obtained if desired, by melting conventional casting alloys such as are disclosed in U.S. Pat. No. 2,951,757, patented Sept. 6, 1960, and US. Pat. No. 3,164,465, patented Jan. 5, 1965, the disclosures of each of said patents being incorporated herein by reference. Accordingly, the alloys produced in accordance with the invention include a fully dense consolidated-powder alloy having the composition defined by any of claims 1-6 of US. Pat. No. 2,951,757 or any of the claims 1-9 of US. Pat. No. 3,164,465, wherein the improvement comprises the hot workability of the alloy. The atomized alloy charge is then rapidly quenched to solidify the molten particles and prevent appreciable formation of coarse crystals of dispersed secondary phase in the resultant powder. The prealloyed powder is then compressed and mechanically hot worked to consolidate the powder into metal stock having a density substantially equivalent to the alloy in its cast state.

Nickel-base alloys with the composition set out in Table I were made according to this invention and compared with prior art alloys of nearly identical chemical composition.

of atomized powder and on each consolidated and annealed plate. Photomicrographs were taken at 1000 so that microstructural comparisons could be made between commercially produced and atomized and consolidated alloys. Tensile specimens were taken from the centermost portion of the forged and rolled plate which represented the area of densest material. Room temperature and elevated temperature tensile tests at temperatures from 1400 to 2000 F. were performed in a vacuum of better than 1X10- torr at a strain rate of 0.05 inch per minute. Material for the test specimens was machined into sheet tensile specimens approximately 2 inches long (1 inch gauge length) 0.125 inch thick, and 0.5 inch wide (0.20 inch in gauge section).

The specimens were tested in the as-rolled condition without heat treatment, Ultimate tensile strength, 0.2% offset yield strength and percent elongation in a 1.0 inch gauge length, were determined from load-elongation curves and from measurements of the gauge length scribe marks on the broken tensile specimens. The results of these tests appear in Table II.

TABLE I TABLE II (I) (II) Tensile strength, 0.27 yield Percent Alloy p.s.i strength p.s.i. elongation Prior art Prior art EL- s Mar-M-200 EL-45 Nicrotung EL-9 230, 000 183, 000 7 Prior art (cast) 135, 000 120, 000 7 0.12-0.17 0.15 0. 008-0. 13 0.10 E 5 210,000 183,000 4 9-11 10 9-11 10 Prior art (cast) 130, 000 120,000 a 8-10 0 11-13 12 4. 75-5. 25 5 a. 75-1. 75 4 1. 75-2. 25 2 a. 75-4. 75 4 11.5-13 5 12.5 7-8.5 s

I It will be seen from the foregoing table that at room Q05 (L05 temperature the tensile and 0.2% yield strengths of the 0.01-0.02 0. 013 0.02-0.08 0. 05 atomized and consolidated alloys of the present invention 0. 75-1. 25 1 Balance Balance Balance Balance are appreciably greater than the tenslle and 0.2% yield Alloys to be atomized were heated to a temperature of about 200 to 300 F. above fusion temperature in an induction-heated magnesia crucible under an argon blanket. The molten metal was poured into a preheated zirconialined tundish which has a A in opening in the bottom. The narrow stream of the molten alloy charge from the tundish passed through the center of a mold steel, zirconia lined nozzle of inch diameter opening and was atomized by a jet of high pressure (350 p.s.i.) argon just below the tip of the nozzle. The droplets of prealloyed atomized alloy were rapidly quenched by the inert gas and by a large reservoir of water in the bottom of the atomizing chamber.

The atomized powder obtained was washed several times with acetone, dried and screened to +80 and -80 mesh size fractions. The --80 mesh portion was placed in an Inconel can for consolidation. The can was lined with a sheet of molybdenum to prevent bonding between the can and the powder during consolidation and to facilitate removal of the can material after fabrication. The powders were packed into the can on a vibrating table to obtain as much settling as possible and then cold pressed at from 15 to 30 t.s.i. After the lids were welded on, the cans were hammer forged from approximately 2 inches down to 0.5 inch in height at a temperature of approximately 2125 F. Following reheating the resultant plate was hot rolled at the same temperature down to inch plate using a 10% reduction in thickness for each rolling pass. The consolidated material was then air cooled to room temperature, the plates were annealed, the canning material stripped away and the wrought material sectioned for testing.

Metallographic observations were made on each batch strengths of cast alloys produced in the conventional manner. Metallographic observations showed that dendrite spacing of these atomized superalloys is significantly refined in comparison to conventionally melted and cast superalloys. For example, experimental alloy EL-45 showed a secondary dendrite arm spacing of 0.00008 inch in comparison to a conventionally melted and cast superalloy such as Nicrotung having a dendrite arm spacing of 0.005 and above.

In elevated temperature tests the atomized and consolidated EL-96 demonstrated tensile and yield strengths at 1400 F. vastly superior to the cast alloy Mar-M-200. Further, Mar-M-200 in the cast state has an 1800 F. ductility of 4.5% while the atomized and consolidated form (EL-96) was found to have an elongation of 27.0%.

What is claimed is: I

1. A fully dense consolidated-powder alloy consisting essentially of:

carbon about 0.08% to 0.20% cobalt about 9% to 34% chromium about 8% to 17% aluminum about 2.5% to 6.0% titanium about 1.7% to 5% tungsten about 6.5% to 14% silicon about 0% to 0.2% manganese about 0% to 0.2% iron about 0% to 2 zirconium about 0% to 0.09% boron about 0% to 0.08% columbium about 0% to 1.5%

and the balance nickel with usual impurities in ordinary amounts, said alloy being characterized by being hot workable.

2. An alloy having the composition of the alloy of claim 1, prepared by consolidation of a powder in which the dendrite arm spacing is less than about 0.0003 inch.

3. An alloy according to claim 1, consisting essentially of:

and the balance nickel with usual impurities in ordinary amounts.

4. A fully dense consolidated-powder alloy consisting essentially of:

carbon about 0.15% cobalt about chromium about 9% aluminum about 5% titanium about 2% tungsten about 12.5% zirconium about 0.05% boron about 0.013% columbium about 1% and the balance nickel with usual impurities in ordinary amounts, said alloy being characterized by being hot workable.

5. An alloy according to claim 1, consisting essentially of:

6 carbon about 0.08% to 0.13% cobalt about 9% to 11% chromium about 11% to 13% aluminum about 3.75% to 4.75% titanium about 3.75% to 4.75 tungsten about 7.0% to 8.5% zirconium about 0.02% to 0.08% boron about 0.02% to 0.08%

and the balance nickel with usual impurities in ordinary amounts.

6. A fully dense consolidated-powder alloy consisting essentially of:

carbon about 0.10% cobalt about 10% chromium about 12% aluminum about 4% titanium about 4% tungsten about 8% ziconium about 0.05 boron about 0.05

and the balance nickel with usual impurities in ordinary amounts, said alloy being characterized by being hot workable.

References Cited UNITED STATES PATENTS 3,524,744 8/1970 Parikh -171 0 RICHARD O. DEAN, Primary Examiner US. Cl. X.R.

29-182; 75-05 BB, 0.5 C 

